Method and apparatus for correlating signalling in a communications network

ABSTRACT

A node in an IMS network receives circuit switched signalling relating to a communication session sent from a terminal over a circuit switched access network, and also receives packet switched signalling relating to the communication session sent from the terminal over a packet switched access network. The packet switched signalling comprises an identifier, and the node uses the identifier to correlate the circuit switched signalling with the packet switched signalling. This allows terminals using ICS to send signalling over both packet switched and circuit switched access networks, and a receiving node to correlate those signals.

TECHNICAL FIELD

The invention relates to the field of communications networks, and inparticular to the field of IP Multimedia Subsystem Centralized Servicesnetworks.

BACKGROUND

The IP Multimedia Subsystem (IMS) is the technology defined by the ThirdGeneration Partnership Project (3GPP) to provide IP Multimedia servicesover mobile communication networks. IP Multimedia services provide adynamic combination of voice, video, messaging, data, etc. within thesame session. As the number of basic applications, and the media whichit is possible to combine, increases, so will the number of servicesoffered to the end users, giving rise to a new generation ofpersonalised, rich multimedia communication services. The IMS is definedin the 3GPP Specification 23.228.

The IMS makes use of the Session Initiation Protocol (SIP) to set up andcontrol calls or sessions between user terminals (or user terminals andapplication servers). The Session Description Protocol (SDP), carried bySIP signalling, is used to describe and negotiate the media componentsof the session. Whilst SIP was created as a user-to-user protocol, IMSallows operators and service providers to control user access toservices and to charge users accordingly.

FIG. 1 illustrates schematically how the IMS 3 fits into the mobilenetwork architecture in the case of a GPRS/PS access network. As shownin FIG. 1 control of communications occurs at three layers (or planes).The lowest layer is the Connectivity Layer 1, also referred to as thebearer, or traffic plane and through which signals are directed to/fromuser terminals accessing the network. The GPRS network includes variousGPRS Support Nodes (GSNs) 2 a, 2 b. A gateway GPRS support node (GGSN) 2a acts as an interface between the GPRS backbone network and othernetworks (radio network and the IMS network). A Serving GPRS SupportNode (SGSN) 2 b keeps track of the location of an individual MobileTerminal and performs security functions and access control. Access tothe IMS 3 by IMS subscribers is performed through an IP-ConnectivityAccess Network (IP-CAN). In FIG. 1 the IP-CAN is a GPRS networkincluding entities linking the user equipment to the IMS 3 via theconnectivity layer 1.

The IMS 3 includes a core network 3 a, which operates over the ControlLayer 4 and the Connectivity Layer 1, and a Service Network 3 b. The IMScore network 3 a includes nodes that send/receive signals to/from theGPRS network via the GGSN 2 a at the Connectivity Layer 1 and networknodes that include Call/Session Control Functions (CSCFs) 5. The CSCFs 5include Serving CSCFs (S-CSCF) and Proxy CSCFs (P-CSCF), which operateas SIP proxies within the IMS in the middle, Control Layer.

At the top is the Application Layer 6, which includes the IMS servicenetwork 3 b. Application Servers (ASs) 7 are provided for implementingIMS service functionality. Application Servers 7 provide services toend-users on a session-by-session basis, and may be connected as anend-point to a single user, or “linked in” to a session between two ormore users. Certain Application Servers 7 will perform actions dependentupon subscriber identities (either the called or calling subscriber,whichever is “owned” by the network controlling the Application Server7).

IMS relies on Internet Protocol (IP) as a transport technology. Using IPfor voice communications, however, presents some challenges, especiallyin the mobile community where Voice Over IP (VoIP) enabled packetswitched (PS) bearers may not always be available. To allow operators tostart offering IMS-based services while voice enabled PS-bearers arebeing built out, the industry has developed solutions that use existingCircuit Switched (CS) networks to access IMS services. These solutionsare referred to as IMS Centralized Services (ICS). ICS is also the nameof the Work Item in 3GPP Release 8 addressing these matters.

Currently, ICS contains three alternative solutions known as I1-CS,I1-PS and “N-ICS”. N-ICS is a solution in which the network implementsan adaptation function between a Global System for Mobile Communications(GSM) terminal and the IMS system. The terminal is not required to haveany special functionality, but the CS network requires an update withnew functionality in Mobile Switching Centres (MSC)/Visitor LocationRegisters (VLRs).

I1-CS and I1-PS both require ICS functionality in an accessing terminal.The CS network is on the other hand unaffected and transparent to thecommunication taking place between the ICS capable terminal and an IMSnetwork. IMS CS Control Protocol (ICCP) is a control protocolstandardized in 3GPP to allow an ICS capable terminal to communicatewith service implementations in an IMS network when a CS network is usedas a transport network. It is used for mid-call signalling for servicessuch as call hold and call waiting. Unstructured Supplementary ServicesData (USSD) carries the ICCP protocol transparently from an ICS terminalthrough the CS network to an “IMS-Adapter” that translates ICCP intoSIP. The IMS-Adapter is a new functional entity in the IMS network,termed an IMS CS Control Function (ICCF).

A difference between I1-CS and I1-PS is that I1-PS uses SIP signallingover a PS access network for all ICS related signalling. This includesmid-call manipulations (for example hold/retrieve, Explicit CallTransfer), the addition of non-speech media to an existing call, IMSRegistration, and so on. I1-CS, on the other hand, uses the CS (ICCPover USSD) access network for some purposes such as mid-callmanipulations but not for other actions such as IMS Registration, theaddition of media to calls, and multiparty calls.

FIG. 2 herein illustrates signalling principles for the I1-CS solution.A terminal 8 is shown that connects to an IMS network 3 via a PS accessnetwork 9 and a CS access network 10. Signalling from both accessnetworks traverses an ICCF 11 in the IMS network 3. For both I1-CS andI1-PS, the PS access network 9 is either not suitable for, or notallowed to carry speech (shown as limited capability in FIG. 2), butsuitable for SIP signalling. If the signalling takes placesimultaneously with a CS voice call, Dual Transfer Mode (DTM)capabilities (the ability to have PS and CS bearers simultaneously in 2G(GSM/EDGE)) are required.

The two different signalling sessions shown in FIG. 2 relate to the samecall and should be presented to a remote end (another terminal) as asingle session. A problem with dividing signalling over two protocolsand access networks is that it can be difficult to correlate each set ofsignals to one another. Nodes within the IMS network should be able tomatch signalling sent over a PS access network with signalling sent overa CS access network when the two sets of signalling relate to the samecommunication or session.

SUMMARY

According to a first aspect of the invention, there is provided a methodfor correlating signalling in a communications network. A node in an IMSnetwork receives circuit switched signalling relating to a communicationsession sent from a terminal over a circuit switched access network, andalso receives packet switched signalling relating to the communicationsession sent from the terminal over a packet switched access network.The packet switched signalling comprises an identifier, and the nodeuses the identifier to correlate the circuit switched signalling withthe packet switched signalling. This allows terminals using ICS to sendsignalling over both packet switched and circuit switched accessnetworks, and a receiving node to correlate those signals. By “circuitswitched signalling”, and “packet switched signalling”, it is meantsignalling sent over a circuit switched or a packet switched networkrespectively.

As an option, after establishing the communication session over thecircuit switched access network, the identifier is sent from the node tothe terminal. The identifier is then included in all subsequent packetswitched signalling relating to the communication session. This allowsan identifier to be allocated for each communication session, whichallows the terminal to be able to participate in more than onecommunication session at a time. The identifier is optionally sent tothe terminal in response to a request received from the terminal.

Alternatively, the identifier comprises a value and the terminal'sMobile Subscriber ISDN E.164 number. The identifier is therefore uniqueto the terminal and generated by the terminal, which simplifiessignalling, but means that the terminal can only be involved in onecommunication session that requires packet switched and circuit switchedsignalling to be correlated.

There are several ways in which the identifier can be sent from theterminal to the node. These include including the identifier in aFeature Tag sent in Session Initiation Protocol packet switchedsignalling, including the identifier in a body of Session InitiationProtocol packet switched signalling, and included the identifier as aPublic Service Identity router to the node in the IMS network.

In the case where the identifier is a combination of a value and theterminal's Mobile Subscriber ISDN, the value is optionally included in aFeature Tag sent in Session Initiation Protocol packet switchedsignalling, and the terminal's Mobile Subscriber ISDN is included in aP-Asserted Id.

Optionally, the identifier is correlated with Unstructured SupplementaryServices Data dialogue in the circuit switched signalling.

According to a second aspect of the invention, there is provided amobile terminal for use in a communications network. The terminalcomprises means for sending, to a node in an IP Multimedia Subsystemnetwork, circuit switched signalling relating to a communication sessionover a circuit switched access network. The terminal further comprisesmeans for sending, to the node in the IP Multimedia Subsystem network,packet switched signalling relating to the communication session over apacket switched access network. The packet switched signallingcomprising an identifier for use in correlating the packet switchedsignalling with the circuit switched signalling. This has the advantagethat the receiving node can use the identifier to correlate the circuitswitched signalling with the packet switched signalling.

Optionally, the terminal comprises a receiver for receiving anidentifier from the node in the IP Multimedia Subsystem network. Thisallows an identifier to be allocated for each communication session.Alternatively, the terminal comprises means for generating theidentifier, the identifier comprising a value and the terminal's MobileSubscriber ISDN E.164 number. This simplifies signalling, but onlyallows the terminal to participate in one communications session thatinvolves both packet switched and circuit switched signalling at a time.

According to a third aspect of the invention, there is provided an IPMultimedia Subsystem CS Control Function node for use in acommunications network. The node comprises means for receiving circuitswitched signalling relating to a communication session sent from aterminal over a circuit switched access network, and means for receivingpacket switched signalling relating to the communication session sentfrom the terminal over a packet switched access network. The packetswitched signalling comprises an identifier, which the node uses tocorrelate the circuit switched signalling with the packet switchedsignalling.

Optionally, the node further comprises means for generating theidentifier, and a transmitter for sending the identifier to theterminal. This allows an identifier to be allocated for eachcommunication session, allowing the terminal to participate in more thanone ICS communication session at a time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically in a block diagram how an IMS networkfits into the mobile network architecture in the case of a GPRS/PSaccess network;

FIG. 2 illustrates schematically in a block diagram signallingprinciples I1-CS;

FIG. 3 illustrates schematically in a block diagram signallingprinciples according to an embodiment of the invention;

FIG. 4 is a flow diagram showing the basic steps of an embodiment of theinvention;

FIG. 5 illustrates schematically in a block diagram a mobile terminalaccording to an embodiment of the invention; and

FIG. 6 illustrates schematically in a block diagram an IP MultimediaSubsystem CS Control Function according to an embodiment of theinvention.

DETAILED DESCRIPTION

The following description sets forth specific details, such asparticular embodiments, procedures, techniques, etc. for purposes ofexplanation and not limitation. In some instances, detailed descriptionsof well known methods, interfaces, circuits, and devices are omitted soas not obscure the description with unnecessary detail. Moreover,individual blocks are shown in some of the drawings. It will beappreciated that the functions of those blocks may be implemented usingindividual hardware circuits, using software programs and data, inconjunction with a suitably programmed digital microprocessor or generalpurpose computer, using application specific integrated circuitry,and/or using one or more digital signal processors.

Referring to FIG. 3, a communications network comprises an ICS terminal8, a PS access network 9, a CS access network 10, an IMS network 3, andan ICCF 11 located in the IMS network 3. In a high level embodiment ofthe invention, the terminal 8 includes an identifier in all PSsignalling that it sends. The identifier allows nodes within the IMSnetwork 3 to correlate the PS signalling with the terminal or with aparticular session with which the terminal is involved.

Once a call has been established over the CS access network, anidentifier (termed a PS correlation id) is sent from the ICCF 11 to theterminal 8. Where the ICCF 11 allocates the identifier, it may beallocated from a pool of numbers and may additionally be time-stamped.Alternatively, the terminal 8 generates its own identifier. Anyadditional SIP signalling sent from the terminal relating to the callincludes the identifier.

FIG. 4 is a flow diagram illustrating the basic steps of the invention.In step S1 a, the terminal 8 sends signalling relating to a call over aCS network, and also (as shown in step S1 b) sends signalling over a PSnetwork. The signalling sent over the PS network comprises theidentifier. When the signalling is received S2 at the ICCF 11, the ICCFuses the identifier to correlate S3 the signalling sent over the PSnetwork with the signalling sent over the CS network.

There are different ways in which the identifier can be obtained by theterminal and subsequently sent. According to a first embodiment, it ispossible to establish both originating and terminating ICS calls at theterminal without ICCP signalling. ICCP is used when required for theestablished call, for example for mid call services such as call holdand call waiting. Additional SIP signalling is used when required, forexample for adding media to a CS call transferred over PS. In order toavoid unnecessary signalling, an identifier is only provided to theterminal when signalling over the PS network is required. The identifieris then included in all subsequent SIP signalling over the PS accessnetwork. When the ICCF receives a SIP message with the previouslyassigned identifier, it knows which call the SIP signalling is intendedfor.

Once the identifier has been obtained by the terminal, it is transferredto the ICCF in all subsequent signalling over PS. The following areexamples of ways in which the identifier can be transferred over the PSaccess network:

1. Included in a Feature Tag, for example:+gics.PS-Correlation-Id.“value-of-the-assigned-Correlation-Id”;2. In an XML defined body in the SIP message;3. Used as a Public Service Identity (PSI) to route to the ICCF.

Initial filter Criteria (IFC) triggering on the originating side(S-CSCF) is used for alternatives 1 and 2 to invoke the ICCF functionalentity over the ISC interface, and allow the ICCF to correlate the CSand PS signalling procedures with each other. The B-Number of therecipient used to establish the CS call can be used as a Request-URI forthe SIP signalling over PS. In alternative 3, the identifier must be inthe form of a PSI to be routable in the IMS network, either directlyfrom an I-CSCF to the ICCF or via an S-CSCF.

A PSI is an identification of a service in a network that is used to“call the service”, i.e. similar to dialling, for example, an 800 numberin a PSTN. PSI identification is also used in the ICS solution for the“handover phase” of a call between CS and PS access networks, where aVoice Call Continuity Domain Transfer URI (VDI) is used as a PSI.However, when VDI is used as a PSI, it is an order to the DomainTransfer Function (DTF) to transfer the call from IMS to CS. Theidentifier, on the other hand, is only used to tie CS and PS signallingsessions to each other, when a call using the CS access network isalready established at the ICCF. When the PSI option is used, the PSI isplaced in the Request-URI of the SIP message transferred over PS.

This solution allows several calls to co-exist, as identifiers aredynamically assigned to each call or session that the terminal isinvolved in.

According to a second embodiment of the invention, a simplified optionis to allow the terminal to participate in only one call at a time, andassign a static identifier to each ICS terminal. This value needs to bequalified with the Mobile Subscriber ISDN (MSISDN) of the ICS terminalin order for it to be unique.

The identifier can be transferred to the ICCF in any of the followingways:

1. Included in a Feature Tag, for example:+g.ics.PS-Correlation-Id.MSISDN;2. Included in a Feature Tag with the MSISDN transported in theP-Asserted Id;3. In an XML defined body in the SIP message together with an MSISDN;4. As a PSI to route to the ICCF, where the last part of the PSI is thecalling subscriber's MSISDN or in the P-Asserted Id.

As with the first embodiment, IFC triggering on the originating side(S-CSCF) is used for alternatives 1-3. The B-Number of the recipientused to establish the CS call can be used as a Request-URI for the SIPsignalling over the PS network.

For option 4, the identifier must in this case be in the form of a PSIto be routable in the IMS network, either directly from an I-CSCF to theICCF or via an S-CSCF. The principle is similar to how a VDI is used inICS, but the meaning is different. When VDI is used as a PSI, it is anorder to the DTF (Domain Transfer Function) to transfer the call fromIMS to CS. The identifier, on the other hand, is only used to tie CS andPS signalling sessions to each other. When the PSI option is used, thePSI is placed in the Request-URI of the SIP message transferred over PS.

Referring to FIG. 5, there is illustrated a terminal 8. The terminal 8comprises a receiver 12 for receiving signals. The terminal alsocomprises a processor 13 for including an identifier in SIP signallingsent to an ICCF. A memory 14 may be provided in order for the terminalto retain the identifier. The terminal further comprises a transmitter15 for sending SIP signalling to an ICCF, the SIP signalling comprisingthe identifier.

Referring to FIG. 6, there is illustrated an ICCF 11 according to anembodiment of the invention. The ICCF comprises a receiver 16 forreceiving SIP signalling from a terminal, the SIP signalling comprisingan identifier. The ICCF further comprises a processor 17 for correlatingthe identifier in the SIP signalling with ICCP signalling performed overUSSD via the CS network. The ICCF comprises a memory 18 for storing thedata mapping identifiers to CS signalling. According to one embodiment,the ICCFG further comprises a transmitter 19 for sending an identifierrelating to a call to a terminal. In this case, the processor may alsobe used to generate the identifier.

The invention makes it possible to correlate two separate signallingsessions where one signalling session is established over CS (ICCP/USSD)and the other over PS (SIP), where both sessions relate to the samecall.

Although various embodiments have been shown and described in detail,the claims are not limited to any particular embodiment or example. Noneof the above description should be read as implying that any particularelement, step, or function is essential such that it must be included inthe claims' scope. The scope of patented subject matter is defined bythe claims.

The following acronyms have been used in this specification:

24.008 3GPP layer 3 specification for the radio interface

3GPP 3rd Generation Partnership Project

CAMEL Customized Applications for Mobile network Enhanced LogicCAP CAMEL Application Part—a signalling protocol used with CAMEL

CS Circuit Switched DTF Domain Transfer Function DTM Dual Transfer Mode

eDTF enhanced Domain Transfer FunctiongsmSCF GSM Service Control FunctionI1-cs MS CS Control Channel implemented over the CS domainI1-ps IMS CS Control Channel implemented over the PS domain

IA IMS Adaptor ICCF IMS CS Control Function ICCP IMS CS Control Protocol

ICS IMS Centralized Services accessed over CS

IDP Initial Detection Point (CAMEL) IMS IP Multimedia Subsystem INIntelligent Networks

MAP Mobile Application Part—a signalling protocol

MGFC Media Gateway Control Function MMI Man-Machine Interface

MSISDN Mobile Subscriber ISDN (E.164 number)

PS Packet Switched PSI Public Service Identity SIP Session InitiationProtocol TEL Telephony UE User Equipment URI Uniform Resource IdentifierUSSD Unstructured Supplementary Services Data

VDI VCC Domain Transfer URI (VDI)

1. A method for correlating signalling in a communications network, themethod comprising: at a node in an Internet Protocol MultimediaSubsystem network, receiving circuit switched signalling relating to acommunication session sent from a terminal over a circuit switchedaccess network; receiving packet switched signalling relating to thecommunication session sent from the terminal over a packet switchedaccess network, the packet switched signalling comprising an identifier;using the identifier to correlate the circuit switched signalling withthe packet switched signalling.
 2. The method according to claim 1,further comprising, after establishing the communication session overthe circuit switched access network, sending the identifier from thenode to the terminal, and including the identifier in all subsequentpacket switched signalling relating to the communication session.
 3. Themethod according to claim 2, wherein the identifier is sent to theterminal in response to a request received from the terminal.
 4. Themethod according to claim 1, wherein the identifier comprises a valueand the terminal's Mobile Subscriber Internet Services Digital NetworkE.164 number.
 5. The method according to any one of claims 1 to 4,wherein the identifier is included in a Feature Tag sent in SessionInitiation Protocol packet switched signalling.
 6. The method accordingto any one of claims 1 to 4, wherein the identifier is included in abody of Session Initiation Protocol packet switched signalling
 7. Themethod according to any one of claims 1 to 4, wherein the identifier isincluded as a Public Service Identity router to the node in the InternetProtocol Multimedia Subsystem network.
 8. The method according to claim4, wherein the value is included in a Feature Tag sent in SessionInitiation Protocol packet switched signalling and the terminal's E.164number is included in a P-Asserted Id.
 9. The method according to anyone of claims 1 to 8, wherein the identifier is correlated withUnstructured Supplementary Services Data dialogue in the circuitswitched signalling.
 10. A mobile terminal for use in a communicationsnetwork, the terminal comprising means for sending, to a node in anInternet Protocol Multimedia Subsystem network, circuit switchedsignalling relating to a communication session over a circuit switchedaccess network; means for sending, to the node in the Internet ProtocolMultimedia Subsystem network, packet switched signalling relating to thecommunication session over a packet switched access network, the packetswitched signalling comprising an identifier for use in correlating thepacket switched signalling with the circuit switched signalling.
 11. Theterminal according to claim 10, comprising a receiver for receiving anidentifier from the node in the Internet Protocol Multimedia Subsystemnetwork.
 12. The terminal according to claim 10, comprising means forgenerating the identifier, the identifier comprising a value and theterminal's Mobile Subscriber Internet Services Digital Network E.164number.
 13. An IP Multimedia Subsystem Circuit Switched Control Functionnode for use in a communications network, the node comprising: means forreceiving circuit switched signalling relating to a communicationsession sent from a terminal over a circuit switched access network;means for receiving packet switched signalling relating to thecommunication session sent from the terminal over a packet switchedaccess network, the packet switched signalling comprising an identifier;means for correlating the circuit switched signalling with the packetswitched signalling using the identifier.
 14. The node according toclaim 13, further comprising: means for generating the identifier; and atransmitter for sending the identifier to the terminal.